> ## Documentation Index
> Fetch the complete documentation index at: https://auth0.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Enable Automatic Migration to Auth0 from an External User Store
> Gradually migrate users from your external user store to Auth0 as they log in by enabling import mode on your custom database connection.
When using a [custom database connection](/docs/authenticate/database-connections/custom-db), Auth0 supports automatic migration (also known as *trickle migration* or *lazy migration*) of users from your external user store to the Auth0 database.
Using automatic migration, your users are moved to Auth0 the first time they log in and do not need to reset their password as a result of the migration.
When a user authenticates via a custom database connection marked for import to Auth0, the following process takes place:
For migrated users, Auth0 authenticates against the Auth0 database. For unmigrated users, Auth0 executes your custom login script and, on a successful login, migrates the user to the Auth0 database. New users are added directly to the Auth0 database.
## Start the migration
To configure automatic migration, you need:
* A [custom database connection](/docs/authenticate/database-connections/custom-db/create-db-connection) using your external user store.
From [Auth0 Dashboard > Authentication > Database](https://manage.auth0.com/#/connections/database), select the custom database connection.
In the **Settings** tab, toggle **Import Users to Auth0** and select \*\*Save.
[Database action scripts](/docs/authenticate/database-connections/custom-db/templates) are code you write that Auth0 executes when interacting with your external user store, like when a user logs in or signs up.
Next, in the **Custom Database** tab, find the **Database Action Scripts** section.
When you enable automatic migration, you must implement two scripts:
* The [`login()` script](/docs/authenticate/database-connections/custom-db/templates/login), which executes when a user who is not in the Auth0 database tries to log in.
* The [`getUser()` script](/docs/authenticate/database-connections/custom-db/templates/get-user), which executes in scenarios like sign-ups and password changes. The data returned from the `getUser()` script is migrated into the new Auth0 user profile.
## Complete the migration
Verify that the migration is complete by checking that all users in your external user store are in the Auth0 user store.
You can view all users in the Auth0 user store in two ways:
* On [Auth0 Dashboard > User Management > Users](https://manage.auth0.com/#/users).
* With the Management API's [List or Search users endpoint](/docs/api/management/v2/users/get-users) (`GET /users`).
Once you confirm that all users are migrated, you can disconnect your external user store by updating your database action scripts to no operation functions. This prevents Auth0 from connecting to your external user store to authenticate users.
Leave the **Import Users to Auth0** option on. This setting is what tells Auth0 to use the Auth0 user store for your custom database connection (instead of using only your external user store via your database action scripts).
From [Auth0 Dashboard > Authentication > Database](https://manage.auth0.com/#/connections/database), select your database connection. On the **Custom Database** tab, find the **Database Action Scripts** section
Update the **Login** script:
```javascript lines theme={null}
function login (email, password, callback) {
return callback(null, null);
}
```
Update the **Get User** script:
```javascript lines theme={null}
function getByEmail (email, callback) {
return callback(null, null);
}
```
## Troubleshoot user migration issues
If you encounter any issues with automatic migration, first read [Custom Database Connection and Action Script Best Practices](/docs/authenticate/database-connections/custom-db/custom-database-connections-scripts).
### Duplicated user or user already exists
During the user migration process, Auth0 first creates a partial user profile in an internal user store, and then creates a full user profile in your database connection. If an issue occurs that prevents this full user profile from being created, you may encounter a "The user already exists" error.
A `DUPLICATED_USER` error indicates that the user exists in Auth0's internal user store but not in your tenant. You may encounter this error if:
* You try to use more than one migration method (for example, automatic migration then bulk user import).
* You delete a user from your database connection and then try to recreate the user.
* You attempt to create a new user in your database connection when the user already exists in your external user store.
To resolve `DUPLICATED_USER` or "The user already exists" errors:
1. Check the `console.log()` statements with the [Actions Real-time Logs](/docs/customize/actions/actions-real-time-logs).
2. Delete the user with the Management API [Delete a User](https://auth0.com/docs/api/management/v2#!/Users/delete_users_by_id) endpoint.
3. Delete the user with the Management API [Delete a Connection User](https://auth0.com/docs/api/management/v2#!/Connections/delete_users_by_email) endpoint.
4. Instruct the user to log in or change their password to reattempt migration.
### Missing metadata
If a user is interrupted during the login or change password flow that initiates the migration process, Auth0 may not be able to transfer their metadata (`user_metadata` or `app_metadata`) along with their other profile data.
You can mitigate this scenario by creating an [Action](/docs/customize/actions) that verifies the user's profile is missing metadata, retrieves it from the external use store, and then stores it in Auth0.